The present invention relates to the synthesis of wholly aromatic polyesters and particularly to improvements in the processibility of a polymer derived from 4-hydroxybenzoic acid, 2,6-naphthalenedicarboxylic acid, and hydroquinone by incorporation of or less than 25 mole % of a moiety derived from 6-hydroxy-2-naphthoic acid.
Wholly aromatic polyesters are well known in the art and are widely employed in molding applications for the production of engineering plastics. The term "wholly" indicates that each monomer moiety present contributes at least one aromatic ring to the polymer backbone. Although commercially successful, wholly aromatic polyesters tend to be difficult to melt process using conventional melt processing procedures. This is particularly true when such materials are processed by injection molding or melt spinning. The processing difficulties arise from the somewhat intractable nature of such polymers and their relatively high melting points.
U.S. Pat. No. 4,067,852 discloses a wholly aromatic polyester preferably derived from 60 mole % 4-hydroxybenzoic acid, 20 mole % 2, 6-naphthalenedicarboxylic acid, and 20 mole % of hydroquinone. The resulting polymer is reported to possess a liquid crystalline molten state at a temperature of below approximately 330.degree. C.
U.S. Pat. No. 4,083,829 discloses an aromatic polyester consisting of recurring moieties derived from 4-hydroxybenzoic acid, 2,6-naphthalenedicarboxylic acid, and hydroquinone by the addition of meta-linked monomers or side chain groups such as isophthalic acid and resorcinol. The polyester is reported as being not intractable and may be melt extruded at temperatures below approximately 300.degree. C. and preferably below approximately 280.degree. C. However, use of meta-linked and side-chain groups may cause intrachain motions that result in poor physical properties at temperatures above which intrachain motions occur because the linearity of the polymer is disrupted and becomes more amorphous.
Further means of improving the processability of the wholly aromatic polyester of the '852 patent is by incorporation of meta-oxybenzoyl moieties (U.S. Pat. No. 4,130,545) or addition of methyl hydroquinone (U.S. Pat. No. 4,337,191). Both of these patents report a melting temperature reduction for preferred embodiments in the vicinity of approximately 300.degree. C.
U.S. Pat. No. 5,025,082 issued to Kishiro, et al. discloses a wholly aromatic polyester and aromatic polyester amide directed toward having melting points low enough for use in conventional polyester producing apparatuses, but not so low that the polymer becomes amorphous. Specifically, this reference cites an aromatic polyester comprised of moieties from 2,6-naphthalenedicarboxylic acid, 4,4'-dihydroxy-diphenyl, an aromatic diol, and p-hydroxybenzoic acid. This composition is reported as possessing a liquid crystalline molten state at a temperature of 320.degree. C. or lower.